For the Donuts
Pre-heat oven to 325 degrees and lightly grease or cooking-spray the donut pan (or muffin tin).
In a large bowl, mix together flour, sugar, baking powder, cinnamon (and salt).
Add buttermilk, egg, vanilla and melted butter. Whisk or stir to combine.
Use a pastry bag fitted with a large round tip (or a Ziploc bag with a corner cut off) to pipe the batter into a 6-donut pan. Or just be extra neat and do this with a spoon which is what I did.
Bake 8 to 11 minutes in the preheated oven, until doughnuts spring back when touched or until dough is set. Donuts will not be golden brown, but should be springy. (I always underbake my baked goods but in my oven these took 10-11 minutes to set up)
Allow to cool slightly before removing from pan, about 5 minutes.
For the Glaze
While donuts are baking or cooling, make the glaze by combining powdered sugar, vanilla extract, and liquid of your choice. Add the water or milk, one teaspoon at a time, until desired consistency is achieved.
Dip the donuts into the glaze while warm (or cool).
Add sprinkles or other extras at end as desired.
To make Gluten Free: use your favorite gluten free flour blend